Gas sensors are used in a variety of applications that require qualitative and quantitative analysis of gases. For example, gas sensors have been used for many years in automotive vehicles to sense the presence of oxygen in exhaust gases, such as when an exhaust gas content switches from rich to lean or lean to rich. In automotive applications, the direct relationship between oxygen concentration in the exhaust gas and air-to-fuel ratio of the fuel mixture supplied to the engine allows the gas sensor to provide oxygen concentration measurements for determination of optimum combustion conditions, maximization of fuel economy, and the management of exhaust emissions.
A conventional stoichiometric gas sensor typically consists of an ionically conductive solid electrolyte material, a porous platinum electrode with a porous protective overcoat on the sensor's exterior, which is exposed to the exhaust gases, and a porous electrode on the sensor's interior surface exposed to a known oxygen partial pressure. These electrochemical devices need to be heated to a minimum temperature about 250° C. to 300° C. before they become active, so are usually designed to include an electrically powered heater rather than rely solely on heat from exhaust gas.
Sensors typically used in automotive applications use an yttria-stabilized, zirconia-based electrochemical galvanic cell operating in potentiometric mode to detect the relative amounts of oxygen present in an automobile engine's exhaust. When opposite surfaces of this galvanic cell are exposed to different oxygen partial pressures, an electromotive force is developed between the electrodes on the opposite surfaces of the zirconia electrolyte, according to the Nernst equation:   E  =            (              RT                  4          ⁢          F                    )        ⁢          ln      ⁡              (                              P                          O              2                        ref                                P                          O              2                                      )            where:
E=electromotive force
R=universal gas constant
F=Faraday constant
T=absolute temperature of the gas
PO2ref=oxygen partial pressure of the reference gas
PO2=oxygen partial pressure of the temperature of the exhaust gas
Due to the large difference in oxygen partial pressures between fuel rich and fuel lean exhaust conditions, the electromotive force changes sharply at the stoichiometric point, giving rise to the characteristic switching behavior of these sensors. Consequently, these potentiometric gas sensors indicate qualitatively whether the engine is operating fuel rich or fuel lean, without quantifying the actual air to fuel ratio of the exhaust mixture. Increased demand for improved fuel economy and emissions control has necessitated the development of gas sensors capable of quantifying the exhaust oxygen partial pressure over a wide range of air fuel mixtures in both fuel-rich and fuel-lean conditions. These sensors may have multiple cells, one or more of which is operated amperometrically in conjunction with a gas diffusion barrier to generate a diffusion limited output. The oxygen reference gas may be obtained via an air channel built into the sensor element, and/or pumped electrochemically through the electrolyte from the exhaust gas. Additionally, faster light-off time for the sensor (time to activity) is important for emissions control, as emissions are at the highest levels at startup. Devices with integral heaters have been developed to decrease light-off time.
Gas sensors with a pumped oxygen reference need a stable current to pump oxygen from the exhaust to the oxygen reference. The output of the active zirconia is dependent on having a consistent current of oxygen pumped into the oxygen reference (greater than 7 microamps). Too high a current level caused by a high voltage on the zirconia will cause the output to shift and higher levels will permanently damage the zirconia. A very low current level will cause the zirconia output to shift in the other direction and a sufficiently low current level may allow contamination of the oxygen reference causing a drastic shift in output. The voltage to drive this pumping current may be diverted from a heater through a pumping resistor, or it may be provided by a voltage source in an electronic controller. However, if the resistivity of the alumina body is too low, the leakage current can exceed the current through the pumping resistor. Also, excessive leakage current from the heater can add noise to the sensor signal and contribute to erroneous sensor output.
There exists a need in the art for a higher resistivity tape to reduce electrical leakage during sensor operation.